Hi Wayne

I agree with peanut, I also have this variety of acer, mine is ok at the moment but another one has scab which I have been treating. I remember reading that varigated varieties are prone to reverting to type, spotted laurel occassionally does this.
I wonder if you cut away the reverted bit and pot up as a cutting the other may come true. If not 2 for the price of one can't be bad.

I only found out recently that acers can be taken from cuttings so you may be sucessful.
Good Luck